Leadframes have been commonly used in the packaging of integrated circuit dies because of their versatility and low cost. Typically, the leadframe is planar in construction and surrounds the integrated circuit semiconductor die. The leadframe is typically formed by stamping or etching a predetermined pattern of leads in a sheet of metal. The leadframe so processed typically includes at least two arrays of leads on opposite sides of a central area for accommodating the die. The leads typically have inner lead portions close to the die area for connection to the die and outer lead portions extending away from the die area. As described below, the leads are usually arranged so that the leads radiate from their outer lead portions inward towards the central die area to their inner lead portions.
Bonding wires are affixed to the inner lead portions and to selected contact pads on the die to electrically connect the die to the leads. The die is attached to and supported by a base (also known as a pad or paddle) which may simply be a portion of the leadframe where the portion is known as a die attach pad. To accommodate a large number of connections between the die contact pads and the leads, the pattern of leads formed in the leadframe is such that the leads form a converging pattern towards the die area where the cross-sectional dimensions and the spacing between adjacent leads become smaller from the outer lead portions towards the inner lead portions, so that the cross-sectional dimensions of the leads and the lead spacings are the smallest at the tips of the inner lead portions immediately adjacent to the die. Such conventional leadframe structure is illustrated in FIG. 1 of U.S. Pat. No. 4,774,635 to Greenberg et al.
The above-described standard packaging scheme utilizing leadframes generally has been satisfactory. However, with the advent of very large-scale integration, the pad pitch on semiconductor dies has been continually reduced. When this happens, the limitations of the above-described conventional packaging scheme become apparent. In the process for forming the leadframe by means of either stamping or etching as described above, the limitation of the stamping and etching processes sets a lower limit to the possible minimum widths of the leads and the spacing between the leads. Typically, the leads of a 6-mil thick (such as in PQFP) leadframe must be at least 6 mils wide and the minimum spacing between adjacent leads is 4 mils. Because of these minimum dimensions which must be maintained for the minimum dimensions of lead widths and lead spacings, in order to accommodate a large number of leads, the inner lead portions must end further away from the die than in previous designs with fewer lead connections. In other words, the bonding wires used to connect contact pads on the die and the inner lead portions of the leads must correspondingly be longer than those used previously.
If the bonding wires are too long, however, they tend to sag and sway, which increases the chances that adjacent bonding wires may contact to create electrical shorts. Sagging wires also increase the stresses at the joints from the bond wires to the leads at one end and to the die contact pads at the other. This increases the probability that wires will break at the joints. Bonding wires used therefore should not be of excessive length. In general, it is desirable to have the wires spanning less than 150 mils between the die contact pads and the leads. The same difficulties will be present where the size of the die is smaller than the standard chip sizes.
Various solutions have been proposed to solve the above-described problems. In U.S. Pat. No. 4,754,317 to Comstock et al., for example, a bridging member of annular square configuration is used to bridge the wide gap between the die and the inner lead portions. The bridging member has thereon transverse plated spaced conductive pathways. A first series of short bonding wires connect selected die contact pads to the inner ends of selected conductive pathways and a second concentric series of bonding wires connect the outer ends of the selected conductive pathways of the bridging member to selected ones of the inner leads of the leadframe. Such scheme is disadvantageous since an extra series of bonding wires is required. As is known to those skilled in the art, precisely affixing bonding wires to closely packed locations is time-consuming and difficult. Having to employ an extra series of bonding wires affixed to closely packed locations as required by Comstock et al. magnifies the problem.
Greenberg et al. proposes another solution in U.S. Pat. No. 4,774,635. In this solution, in order to bridge the wide gap between the die and the increased number of leads of the leadframe, conductive fingers backed by an insulating tape are aligned and bonded to the ends of the fingers of leads on the leadframe. The tape fingers are electrically coupled to the bond pads on the semiconductor die by wire bonding. In other words, the locations and the widths of the tape fingers must be such that these fingers are precisely aligned with the ends of the lead fingers of the leadframe. This requires an accurate alignment process. Furthermore, since the tape fingers must be precisely aligned with the lead fingers of the leadframe, the tapes have to be custom made for each type of leadframe. In other words, different types of tapes must be kept in stock for use with different types of leadframes, which is cumbersome and expensive.
Yet another solution is proposed by Smith et al. in U.S. Pat. No. 4,870,224. Smith et al. proposes the use of a ceramic substrate for supporting a semiconductor die and for bridging the gap between the leads of a leadframe and the contact pads on the die. Contacts are provided on the ceramic substrate to mate with the contact pads of the integrated circuit devices on the die. Conductive lines then couple the contacts on the ceramic substrate to the peripheral edges of the substrate. The ends of the conductive lines at the peripheral edges of the substrate then mate with a set of ends of the leads of a leadframe.
As discussed above, given the current state of the art in stamping or etching leadframes, the minimum spacing between adjacent leads of a leadframe is 4 mils. Since the leadframes are connected to the conductive lines on the ceramic substrate only at the peripheral edges of the substrate, the spacing between adjacent connection sites between the conductive lines and the leads also must be at least 4 mils. If conventional bonding tools are used for connecting the conductive lines to the leads, the bonding operation using the tool requires certain minimum space in order to perform the bonding operation. When the bonding sites are too close together, the conventional bonding tool cannot be used, and expensive single point welding equipment is required. Alternatively, conductive adhesive may be used in an expensive process.
Furthermore, in the design proposed by Smith et al., in order to connect the end portion of each lead to a peripheral edge of the ceramic substrate, the end portion of the lead comprises an upper finger and two lower fingers wherein the peripheral edge is disposed between the upper and lower fingers. As noted above, the stamping and etching processes for forming leadframes imposes lower limits for the widths of leads that can be formed to about 6 mils. This means that the three fingers of the end portion of each lead in Smith et al. must also be at least about 6 mils in width. Therefore, using Smith et al.'s design, each connection between a lead of the leadframe and a conductive line would occupy an extra 12 mils of space at the peripheral edge of the substrate more than it would require in other designs; this vastly reduces the number of connections possible between the die and the leadframe.
None of the above-described proposed solutions is entirely satisfactory. It is therefore desirable to provide an improved die-to-leadframe interconnect assembly at which the above difficulties are alleviated.